Configure and document Elasticsearch Snapshot Lifecyle Management (SLM)

[rpcross]

Configure SLM, backups of the Elasticsearch index in the event the index is corrupted and needs to be restored. This will require setup of a cloud based snapshot repository. Document Mail Archive config of SLM and how to restore snapshots.

See: https://www.elastic.co/guide/en/elasticsearch/reference/7.17/snapshot-lifecycle-management.html https://www.elastic.co/guide/en/cloud/current/ec-aws-custom-repository.html